Balance of Power in Ad Industry at Stake as Google, Microsoft Seek to Control Web Tracking

The end could be near for cookies, the tiny pieces of code that marketers deploy on Web browsers to track people's online movements, serve targeted advertising and amass valuable user profiles.

In the past month, Microsoft Corp., Google Inc. and Facebook Inc. have said they are developing systems to plug into and control this river of data in ways that bypass the more than a thousand software companies that place cookies on...